This is the combination of two RR, now that FindDocBookXML.cmake/FindDocBookXSL.cmake have been moved to kdoctools:
- https://git.reviewboard.kde.org/r/115876/
- https://git.reviewboard.kde.org/r/115879/

Combination of few commits:
1) More generic structure for FindDocBookXML
    
- allows modules to request a generic version of DocBook XML
- keep compatibility variables for now
(originally from 115876)
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2) Rename as FindDocBookXML4, it will be used only for DocBook 4.x
    
The rename reflects the fact that it is used for DocBookXML4 only;
a future DocBookXML5 could be added if needed.
(originally from 115876)

Use the new module name in CMake files.
(originally from 115879)
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3) New DocBookXML4_* variables, explicitly require the DocBookXML4 version
(originally from 115879)
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4) Move DocBookXML4 DESCRIPTION and URL into the cmake file
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5) Move DocBookXSL DESCRIPTION and URL into the cmake file

Compiles, documentation is generated.